The present invention deals with fracturing geologic formations in the region of hydrocarbon bearing zones in order to stimulate production. More specifically, it deals with an improved hydraulic fracturing fluid which has improved rheological properties for delivering a proppant into fractures in order to maintain them in a highly permeable condition for improved hydrocarbon recovery. The invention further relates to the method of preparation and use of the fluid.
Hydraulic fracturing of oil bearing formations has been practiced commercially since 1948. This involves pumping a fluid at a sufficiently high pressure and volumetric rate into the hydrocarbon bearing zone to cause cracks to form and propagate. A granular solid material usually termed a proppant is carried along with the fluid so that it wedges in the thus formed cracks and maintains them in an open condition after the pressure is released. This can greatly increase the permeability, especially of very tight formations, and enables much higher flow rates and greater recovery of the contained hydrocarbons.
At the present, hydraulic fracturing has become a largely predictable practice so that orientation and lengths of cracks can be predetermined and even controlled to a great extent.
Both oil based and water based fracturing fluids are employed, although the latter type is by far the more common. The present invention is directed to water based types and the following discussion is specific to this type.
Fracturing fluids usually contain a number of ingredients. The most important are the gellant, which controls rheological properties, and the proppant. The gellant must maintain the proppant in suspension during fluid preparation, pumping, and ultimate distribution into the cracks in the hydrocarbon bearing formation. In order to do this effectively it must be compatible with a number of greatly differing shear conditions. It is not uncommon for several hundred thousand gallons of fracturing fluid to be injected into a well at pumping rates as high as 50 barrels (42 gal/bbl) per minute. Ideally viscosity should be low when mixing and pumping in order to minimize pumping energy required. However, it must be high enough so that the proppant does not fall out of suspension until it is ultimately delivered to its desired location. Viscosity reduction due to high temperatures in the hydrocarbon bearing zone can further complicate the picture.
In order to accommodate the above conflicting viscosity requirements the gellant will normally convey thixotropic properties to the fracturing fluid. The higher viscosity at lower shear rates will help maintain the proppant in suspension while the lower viscosity experienced at higher shear conditions will improve flow rate and proppant transport conditions. Very typically a crosslinking agent will be used to cause a significant viscosity increase in the region of the bottom of the hole, after the fluid has passed through the great bulk of the well casing.
Ultimately, it is necessary to remove the transport fluid after the proppant is in place in the fractures. There are a number of ways of accomplishing this end but the use of oxidizing agents and enzymes that attack the gellant are most common.
Two references might be cited that give a general background of hydrofracturing. The Petroleum Engineering Handbook, H. B. Bradley, ed., Society of Petroleum Engineers, Richardson, Tex. Chap. 55 (1987) has a very useful general discussion. Specific chemicals used in fracturing fluids are covered in more detail in Chemicals in Petroleum Exploration and Production II, North American Report and Forecasts to 1993, Colin A. Houston and Associates, Inc., Mamaroneck, N.Y. (1984).
While the term "gellant" is in common use and will be used herein it should not be taken literally in the sense that these materials form a conventional nonflowing gel. More appropriately they should be regarded as viscosifiers and rheology control agents. Gellants are most usually based on water soluble derivatives of natural polysaccharide materials such as guar gum, cellulose, or xanthan. Most common of these materials is hydroxypropyl guar. Carboxymethylhydroxypropyl guar is another commonly used guar derivative. Among the cellulosics to be mentioned are hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose and hydroxypropylmethyl cellulose. Down hole temperature will have an important influence on the gellant chosen. Hydroxypropyl guar is useful at lower temperatures and carboxymethylhydroxyethyl cellulose at higher temperatures. Hydroxyethyl cellulose and xanthan have maximum upper temperature tolerance somewhat between these two.
As was noted earlier, crosslinkers are typically used to increase down hole viscosity. Most commonly these are polyvalent metal salts that form chelates; e.g., borates, aluminates, titanates, chromates, zirconates, etc.
Temperature determines the gelbreaking mechanism chosen after the proppant has been delivered. Enzymes are useful up to 50.degree. C. and oxidants such as calcium or sodium hypochlorite and sodium or ammonium persulfate to about 80.degree. C. Heat alone will usually suffice to break the gels by thermal degradation at temperatures above about 135.degree. C.
Proppants are chosen from natural silica sand, usually a material having high roundness in the 10-20 to 20-40 mesh size ranges, and synthetics based on alumina. The latter is preferred when compressive forces will be very high.
Despite 40 years of history and research the ideal rheological characteristics of hydraulic fracturing fluids still can only rarely be obtained. The present invention marks a major step forward in rheological control.